320 SYSTEMATIC DISTRIBUTION [CH. 



hypocotyls are common among Ranunculaceae with concrescent 

 cotyledons, that is to say, among forms which supply indica- 

 tions of the characters of the original Monocotyledonous stock 1 . 



The idea that the Helobieae are descended from a very 

 ancient group of Angiosperms, and have inhabited the water 

 for a correspondingly long period, is ratified by the fact that 

 this series consists of a whole plexus of related families, some 

 of which have departed widely from the original type; it con- 

 tains forms as far asunder, for instance, as Alisma with its many 

 Ranalean features and Naias which represents the very acme 

 of floral reduction. One minor piece of evidence favouring the 

 antiquity of the water habit in the case of the Helobieae, is the 

 fact that this Cohort includes all the marine Angiosperms a 

 biological group which probably originated through the further 

 modification of fresh-water forms. 



That the Nymphaeaceae and the related Ceratophyllaceae 

 on the one hand, and the Helobieae on the other, have taken 

 to aquatic life with such conspicuous success, suggests that the 

 original Ranalean stock, from which they both sprang, may 

 have been particularly well adapted to water life. In the Ranun- 

 culaceae the tendency to aquatic habits in the case of the genus 

 Ranunculus is obvious; besides the definitely aquatic sub-genus 

 Batrachium, the Buttercups include a number of forms, such 

 as R. sceleratus and R. Flammula^ which are capable both of 

 land and water life. The singularly slight difference in general 

 anatomy, between the terrestrial and aquatic species of Ranun- 

 culus^ suggests that the land forms are of a type which does 

 not require great changes of structure in order to succeed in 

 water life. 



It is a remarkable fact that the Sympetalae the most highly 

 evolved group of Angiosperms has produced no entirely 

 aquatic family, nor any single aquatic species which has become so 

 far adapted to water life as to have acquired submerged hydro- 

 philous pollination. The very large family of the Compositae, 

 which may perhaps be classed as the ultimate term of the 

 1 Sargant, E. (1903) and (1908). 



